1. Field of the Invention
The present invention relates to a process for producing a single crystal silicon wafer, particularly a highly thermally-conductive single crystal silicon wafer
2. Related Background Art
Computer performance has rapidly progressed in recent years. Particularly, the processing speed of computers has significantly improved primarily due to the use of microprocessors, which have a high operating frequency. However, the use of such microprocessors results in an increased electric power consumption. For instance, an Alpha 21264 chip produced by Compaq, which chip is presently used as the microprocessor, operates at a frequency of 600 MHz, which is quite high, resulting in a power consumption of 109 W, which is also quite high. Microprocessors with an even higher operating frequency have been developed. These semiconductor chips consume large amounts of electric power when they are operated at a high frequency, leading to a large increase in temperature. As the temperature increases, the mobility of electrons or holes is decreased, deteriorating the performance of the chip as the microprocessor. Also, a temperature increase, which is generally called a “hot spot”, occurs causing critical damages.
In order to prevent these problems from occurring, there has been proposed a way to cool the semiconductor chip, which is used as the microprocessor. However, this not considered to always be a decisive solution. Thus, it is necessary to focus on the thermal conductivity inside the semiconductor chip in order to sufficiently solve the problem.
In the X-ray optical systems, a silicon single crystal has been used as a mirror in which Bragg reflection is utilized. In this technical field, strong X-ray sources, such a as synchrotron, have been developed and their use has started to predominate. In that connection, there has been developed a way of cooling the silicon single crystal so that the damage to the mirror due to heat is prevented even when it is subjected to strong radioactive rays.
Separately, in the field of the photovoltaic power generation, an optical-concentration type photovoltaic power generation system, in which a high performance solar cell fabricated using a silicon single crystal is arranged on a focal plane where the sunlight is concentrated to a magnitude of 10 to 500 times by means of an optical focusing system to generate power, has been spotlighted. In this photovoltaic power generation system, in principle, the photoelectric conversion efficiency is to be inclreased. However, if the solar cell is not sufficiently cooled, the advantage of increasing the photoelectric conversion efficiency by way of supplying highly concentrated light to the solar cell is reduced. Thus, attempts have been made to cool the solar cell in the optical-concentration type photovoltaic power generation system.
However, it is expected that not only the X-ray optical systems, but also the optical-concentration type photovoltaic power generation system will be more improved with respect to performance. In that case, it is presumed that the thermal conductivity of the inside of the silicon single crystal will become an important subject to be thoroughly considered.
Also, in view of this, extensive studies have been conducted in order to find semiconductor materials, which hardly generate any heat and whose characteristics do not easily deteriorate in such applications as described above, and which can replace the silicon semiconductor material used in various devices as described above. However, it is difficult to find an appropriate semiconductor material of this type, because even if it is accidentally discovered, it will be very expensive or harmful to the environment.
Besides, particularly in the case of a device such as a microprocessors, which is fabricated by a complicated process, when a prescribed semiconductor that is ordinarily used is changed to another semiconductor material, it is necessary to have another look at a series of the steps and the materials used in every step of the fabrication process, and this cannot be easily performed in practice.
In order to resolve this situation, U.S. Pat. No. 5,144,409 (hereinafter referred to as document 1) proposes a process of producing a semiconductor device using a silicon enriched with a silicon isotope whose mass number is 28 (that is, 28Si). Document 1 describes that in the naturally occurring form, silicon is primarily composed of three kinds of stable silicon isotopes, namely, 92.2% 28Si, 4.7% 29Si, and 3.1% 30Si. Although it is almost impossible to chemically identify these silicon isotopes, they are different from each other in terms of their mass number. Because of this difference, they influence the thermal oscillation of the atomic nucleus and increase the scattering of photons. Thus, the mixture composed of these three silicon isotopes has an inferior thermal conductivity. In this respect, it is understood that the gist of the technique described in document 1 is directed to improving the thermal conductivity by increasing the content of 28Si.
Research News Release of Max Plank Society (published May 10, 2000) (hereinafter referred to as document 2) indicates that at room temperature, an isotopically pure silicon crystal (enriched to 99.8588% 28Si) is a 60% better heat conductor than natural silicon, which consists of three kinds of stable silicon isotopes (92.2% 28Si, 4.7% 29Si, and 3.1% 30Si). Document 2 also indicates that the thermal conductivity of this silicon crystal is enhanced at a temperature that is lower than room temperature.
As the process to enrich a specific isotope element, U.S. Pat. No. 4,584,073 (hereinafter referred to as document 3) discloses a process for separating an isotope of an element from other isotopes of the element in a compound. In more detail, the process of document 3 comprises depositing this compound as a mono-layer on small glass beads, entraining the coated beads in a carrier gas containing a scavenger gas, exposing the entrained, coated beads to light from a laser that excites the one isotopic compound and causes it to react with the scavenger gas to form a reaction product while the other isotopic compounds remain stable, separating the other isotopic compounds from the reaction product and removing the reaction product from the beads to provide a desired isotope element.
U.S. Pat. No. 5,176,885 (hereinafter referred to as document 4) discloses a process for separating the lower molecular weight isotope fraction from the higher molecular weight isotope fraction of an element, which forms a water soluble acid at a temperature amenable to an ion exchange, where the separation is accomplished by shifting the equilibrium constants of absorption and desorption by temperature variations of the acidic solution. Document 4 also discloses that the process is preferably employed to enrich a boric acid solution in its boron-10 isotope fraction by employing ion exchange resins, which are capable of thermally storing and releasing boron isotopes.
In the case where it is intended to produce a silicon product enriched in 28Si in accordance with a technique as above described, the process becomes unavoidably complicated, because it is necessary for silicon isotopes, whose chemical properties are substantially the same, to be concentrated to separate the desired silicon isotope. This makes the product enriched with the silicon isotope whose mass number is 28 (28Si) considerably expensive.
Now, in general, a single crystal silicon wafer is fabricated by a method wherein an ingot drawn up by the Czockralski method or the floating zone melting method is sliced and then subjected to abrasive polishing. In the case where a single crystal silicon wafer enriched with the 28Si isotope is fabricated by this method, a substantial amount of waste is generated during the slicing process and the surface-polishing process.
Separately, in recent years, there is a tendency to increase the size of a single crystal silicon wafer. Specifically, the diameter of 8 inches is the mainstream in the semiconductor production lines at the present time, and this diameter will be increased to 12 inches in the near future. In order to fabricate a single crystal silicon wafer having such a large diameter, it is necessary to draw up an ingot having a large diameter, using silicon enriched with 28Si, which is costly. This method is difficult to employ on an industrial scale mainly in view of the cost.
Therefore, there have been attempts to produce an epi-wafer by epitaxially growing silicon enriched with 28Si on an ordinary silicon wafer. This method has advantages in that the silicon material utilization efficiency is relatively high and the up-sizing is relatively easy. However, this method is disadvantageous in that because an ordinary silicon wafer whose thermal conductivity is low is used as a substrate. Thus, although this wafer would be effective in improving heat generation uniformity in the chip plane, sufficient improvement cannot be achieved in the heat conduction in the thickness direction.
A gas-phase growth method is often adopted in order to epitaxially grow silicon as described above. In the gas-phase growth method, epitaxial growth is performed by flowing a raw material gas of a silicon compound as silane gas or silicon chloride gas into a reaction vessel and thermally decomposing this raw material gas on a substrate comprising an ordinary silicon wafer arranged in the reaction vessel while being maintained at a high temperature, where a considerable part of the raw material gas is exhausted without being grown on the substrate. In this case, in order to produce an epi-wafer, it is necessary to use a raw material gas of a silicon compound enriched with a particular silicon isotope, which is costly. Also, since a considerable portion of this highly expensive raw material gas introduced into the reaction vessel is exhausted without being used, the epi-wafer produced unavoidable becomes very expensive.
In view of this, it is very advantageous to adopt the liquid-phase growth method as the epitaxial growth method. The liquid-phase growth method is generally performed in the following manner. That is, a metal whose melting point is relatively low, such as indium (In), tin (Sn), gallium (Ga), aluminum (Al) or copper (Cu), is introduced into a crucible where the metal is fused into a liquid as a melt. Thereafter, silicon is dissolved in the melt produced in the crucible until this silicon is saturated in the melt. Then, while supersaturating the silicon dissolved in the melt, for instance, by a method of gradually cooling the melt, a base substrate is immersed in the melt, where the supersaturated silicon is deposited on and is grown on the substrate. In this liquid growth method, only a specific portion of silicon grown on the substrate is taken outside together with the substrate, generating no waste.
However, even in this case, as well as in the case of the gas-phase growth method, a silicon material enriched with a given silicon isotope, which is costly, cannot be used as the substrate in practice. Therefore, it is difficult to make full use of the advantage that the thermal conductivity of an epitaxially grown silicon is sufficiently high.
In this connection, there can be considered a method in which an epi-wafer having an epitaxial layer comprising silicon enriched with a given silicon isotope is provided, and the wafer is ground from the back side until a desired thickness is obtained while leaving the epitaxial layer, which can be achieved by chemical and mechanical grinding or the like. However, in this case, unless the thickness of about 30 μm of the wafer is left, the handling thereof in the following process becomes difficult. In the fabrication of a microprocessor or the like, it will be sufficient for the epitaxial layer to have a thickness of about 10 μm. Nevertheless, this grinding method is insufficient in terms of the cost reduction and also in terms of the improvement of the thermal conductivity.